1. Field of the Invention
The present invention relates to a storage element, a method of manufacturing the same, and a semiconductor storage device.
2. Description of the Related Art
Semiconductor devices including nonvolatile memory cells such as an EEPROM (Electrically Erasable and Programmable ROM), a flash memory and the like are now being used in various electronic devices. To improve reliability such as the number of times of rewriting, data retention resistance and the like of the semiconductor devices and to miniaturize the structure of the semiconductor devices are important challenges.
Resistance change type memories have recently been drawing attention because of advantages thereof from viewpoints of reliability and miniaturization over flash memory structures typified by a floating type already available on the market. The resistance change type memories include ARAM, RRAM, PCRAM, MRAM, Spin RAM and the like. These resistance change type memories are said to be suitable for higher performance and higher degrees of integration in combination of simple structure, high-speed rewriting performance, and multilevel techniques, and are thus drawing attention.
In a nonvolatile memory element of the above-described resistance change type, the smaller the area of contact of a lower electrode with a variable-resistance layer, the more current density is increased and the more an electric field can be concentrated. It is thereby possible to contribute to improvement in rewriting performance and stability of characteristic variations.
However, reduction of the area of contact of the lower electrode with the variable-resistance layer is limited by photolithography performance, which makes miniaturization difficult. In addition, a contact structure of a tungsten-embedded type used commonly and widely has a problem of occurrence of characteristic variations due to a seam (hole) occurring in a central part. Further, it is difficult to increase efficiency of radiation of heat generated when a resistance change is effected by passing a current.
As a reference example, for instance, a projecting electrode object in a line shape projecting from a lower electrode is formed on side walls on both sides of the lower electrode, and a variable-resistance layer (variable resistor) is formed on the upper part of the projecting electrode object. A resistance changing element having an upper electrode formed on the upper part of the variable-resistance layer is disclosed (see Japanese Patent Laid-Open No. 2007-180473, Japanese Patent Laid-Open No. 2007-180474, hereinafter referred to as Patent Documents 1 and 2, respectively, for example). The variable-resistance layer is formed by a method of oxidizing a part of the projecting electrode object, thus limiting choices of resistance material. In addition, because the variable-resistance layer is formed on the upper part of the projecting electrode object formed in a line shape, an electric field tends to concentrate at angular parts of the projecting electrode object as viewed in a plan view, and thus the electric field cannot be uniformly concentrated over the entire region of the part in contact with the variable-resistance layer. Therefore characteristic variations occur easily.